Sequential switching device

ABSTRACT

A sequential switching device for repeating an input signal such as one representing the opening of a circuit breaker at the far end of a cable, to a local switching means. The device includes a normally active on-off transistor oscillator which stops oscillating in a non-conductive condition to warn of a cable fault, and in a conductive condition to repeat the input signal.

United States Patent 1191 Bowen SEQUENTIAL SWITCHING DEVICE [76]inventor: Dennis Dawes Bowen, 8 Rowan Tree Dr., Sale, England [22]Filed: Jan. 26, 1972 [21] Appl. No.: 221,011

[30] Foreign Application Priority Data Jan. 27, 1971 Great Britain3,335/71 May 25, 1971 Great Britain 16,787/71 [52] US. CL... 317/141 S,317/D1G. 10, 317/142 R, 317/146 [51] Int. Cl. Hillh 47/18, l-lOlh 47/22[58] Field of Search 317/ DIG. l0, DIG. 2, 317/146, 141 S, 142 R, 148.5

[56] References Cited UNITED STATES PATENTS Atkins 317/1316. 10

[ Aug. 14, 1973 Atkins 317/D1G. 2 Atkins 3l7/DlG. 2

Primary Examiner-J. D. Miller Assistant Examiner-Harvey FendelmanAttorney-Harold D. Steinberg et al.

[5 7] STRACT A sequential switching device for repeating an input signalsuch as one representing the opening of a circuit breaker at the far endof a cable, to a local switching means. The device includes a normallyactive on-off transistor oscillator which stops oscillating in anonconductive condition to warn of a cable fault, and in a conductivecondition to repeat the input signal.

8 Claims, 2 Drawing Figures PATENTEDMI: 14 ms SHEET 2 OF 2 1 SEQUENTIALSWITCHING DEVICE Prior application ln Great Britain on Jan. 27, 197i andnumbered In Great Britain on May 25, 1971 and numbered The presentinvention relates to fail-safe sequential switching devices. Moreespecially, the present invention provides means for opening a secondswitch if a first switch is opened, as used for tripping an auxiliarycircuit breaker if a main circuit breaker trips. The invention may alsobe used in other ways. For example, if a turbine loses oil it isessential that the associated oil pump is switched off. The circuits ofthe invention are continuously operating and self-checking. Theyindicate whether the components thereof are correctly operational, andthey give an indication or alarm if a fault occurs.

In accordance with the present invention there is provided a sequentialswitching device comprising a solid state switching circuit having an onstate and an off state, and normally alternating between said states toproduce a series of pulses, input terminals connected to said circuitsuch that arrival of an input signal at said terminals holds saidcircuit in one of said states, means for repeating said input signal tooutput terminals when said circuit is held in said one state by saidinput signal, said circuit being held in the other of said states onfailure of a component of said device, and means indicat ing saidfailure when said circuit is held in said other I state.

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings, FIGS. 1 and 2 of which showfail-safe sequential switching circuits in accordance with theinvention.

The circuit shown in FIG. 1 is connected to normally closed contact CT]of an existing first switch or main circuit breaker via input terminals,and is adapted to release contact RAl of a relay RA and opening thus toopen a second switch or auxiliary circuit breaker via output terminalsif contact T1 opens this constituting an input signal. A time delaycapacitor C10 is incorporated in the circuit so that the contact ofrelay RA opens only if contact OTl remains open for a significantperiod.

Relay RA is. normally energised when no alarm condition is present, froma positive voltage supply via resistor R and diode D7. The relay RA anddiode D7 combination is connected in parallel with a solid stateswitching device consisting of a transistor-thyristor combination S4. lfcontact 0T1 opens for more than a significant period set by the delaycircuit Cl0-R20- R21, the thyristor-transistor combination S4 isswitched on (see later). Relay RA is therefore shunted and released andcontact RAl opens.

Under normal conditions, that is, with contact 0T1 closed and hence noinput signal received relay RA is initially fully energised and apositive potential occurs at the junction of resistor R15. and diode D7.This potential charges capacitor C3 via a resistor R14, thereby raisingthe potential of the base of transistor TRC so that it switches on.Current through R16 triggers the thyristor combination S4 on, therebyshunting relay RA. The potential at the junction between resistor R15and diode D7 drops to zero volts, transistor TRC is switched off, andthus switches off the transistor of the thyristor-transistor combinationS4, thereby rendering the thyristor once more non-conductive. Relay RAis thus re-energised. This cycle of pulses repeats at a frequency ofseveral KHz so that relay RA remains oper? ated. It may be rendered slowrelease for this purpose. Consequently contact RAl remains closed. Inaddition, provided there is no fault in the circuit or in the thyristorcombination S4, an AC voltage appears across capacitor C4 which lights alamp L2. This lamp indicates, when lit, that the trip relay circuit isfully operational. If now contact OT]! opens for a significant period,relay RA is short-circuited and releases as described above, its conacttripping the second switch or an auxiliary circuit breaker (not shown)via the output terminals. in addition, current is fed via diode D8 andresistor R21 to keep the lamp L2 alight, this current being normallyshunted by closed 0T1.

Thus the lamp L2 is lit whenever the circuit is correctly functioning.If any of the components fail, the circuit ceases to oscillate and thelamp L2 is extinguished. Additionally, an audible alarm circuit may beconnected to the junction between the lamp L2 and capacitor C4 to givean alarm whenever a fault occurs in the sequential switching circuit.

In a preferred embodiment, resistor R15 has a large resistance comparedwith that of the relay RA. This is so as to limit the current drawn fromthe supply when the relay is short-circuited.

A variable time delay may be provided for relay RA; for example, acapacitor may be connected in parallel therewith. If the capacitor wentopen-circuited, the relay would still fail-safe, but in such a casemight be tripped by a fleeting signal from the contact Tl. If thecapacitor went short circuit, it would shunt the relay, which is again afail-safe condition. Any suitable delay means may be used on the relay.

A second embodiment of the invention is shown in FIG. 2, incorporatingextra safety features.

The circuit shown in this figure is connected to the normally closedcontacts 0T1 of an existing first switch or main circuit breaker and isadapted to open the normally closed contact RAl of a relay RA, and thusto open a second switch or auxiliary circuit breaker when contact OT]opens, this being an input signal. A time delay circuit may beincorporated in the circuit so that contact RAll opens only if contactOTli remains open for a pre-determined period. The double line in thefigure indicates the parts of the circuit necessary to control relay RA.

Relay RA is provided with a normally closed contact RAl for the tripcircuit, and the relay has one terminal at earth potential and the otherterminal connected to a positive supply through a wire-wound resistorR1.

A time delay for release of relay RA can be introduced as by usingshorted turns in the winding, or a slug on its core. Relay RA isnormally energized when no alarm condition is present, from a positivesupply via resistor R1 and diode D1.

A solid state switching device including a transistor T1 is connectedacross the relay RA, and a diode D1 is in series with the relay. Theemitter of transistor T1 is connected to earth via T12. Normally, thebase of transistor T1 is at earth potential so that transistor T1 isswitched off. The normally closed contact 0T1 forms part of a potentialdivider including resistors R8 and R5, such that, in the example, lessthan 3.5 volts ap pears at the point A, and less than 5 milliamps flow 3through contact T1. The circuit is therefore intrinsically safe. TwoZener diodes ZNl and ZN2 are connected in parallel and the parallel.combination thereof is connected between points A and B. in the circuitshown in the example, each of the Zener diodes ZNl and 2N2 has a ratingof 6.2 volts. As two Zener diodes are provided in parallel, the circuitwill fail safe should one or other of the diodes become a short-circuit.An open circuit of one Zener diode (which is the least likely mode offailure of a Zener diode) will likewise not affect the normal circuitoperation.

if contact 0T1 which is normally closed, opens, resistor R6 is added tothe potentiometer, and point A rises to near line potential. Thus, inthe illustrated circuit, 5.8 volts appears across resistor R2 andcapacitor C1. The value of capacitor C1 is chosen so that a significanttime delay may be introduced into the circuit if required. The potentialat point B rises and transistor T1 conducts. Point C is hence connectedto ground potential (Transistor T12 is also conducting, as is describedlater). Relay RA de-energises in a time set by its own delay, and thenormally closed contact RAl opens. Relay RA is also provided with a pairof normally open contacts RA2. These contacts RA2 close when relay RA isde-energised, and a transistor T2 is switched on. This transistor T2then provides a path'from point C to earth which is additional to thatthrough Tl.

When relay RA is energised, and therefore contact RAl is closed andcontact RA2 is open, the base of transistor T9 is earthed and thetransistor is therefore conducting. A further transistor T10 connectedto the collector of transistor T9 also conducts, and an indicating lampL1 is lit. In the example, the lamp Ll may be orange in colour and itsfunction is to indicate when relay RA is energised. lf normally opencontact RA2 should close, as would happen if relay RA were deenergized,line potential is connected to the base of transistor T9, therebyturning off this transistor and hence transistor T10. The lamp L1 isthen extinguished.

If, after tripping, contact OTl re-closes, point A is returned to itsnormal potential which is less than 3.5

volts. Transistor T1 is turned off, but relay RA remains de-energised,as point C is still connected to earth through transistor T2. The relaycan be re-energised by pressing the accept/reset button which turns ontransistor T3, thereby connecting the base of transistor T2 to earth.Transistor T2 is hence tumed off and point C is no longer earthed. RelayRA is therefore re-energised, contact'RAl closes the contact RA2 opens.The lamp L1 is illuminated and-then the reset button is released.

If an open circuit fault appeared between points A and B, or transistorT1 went open-circuit, or point B was short-circuited by any fault toearth, relay RA would be prevented from de-energising upon opening ofcontact 0T1. The circuit is therefore provided with self-checking meansso that a fault in the circuit is indicated whenever it occurs.

A transistor T4 is operated as a known form of relaxation oscillator,the collector of transistor T4 being connected to the base of transistorT]. On switching on, T4 is non-conducting since the base is positivewith the emitter, which starts at ground potential whilst the emitterload'resistor charges-up the condenser. When the emitter becomespositive with respect to the base, T4 conducts. Point B rises fromground potential making the emitter more positive and thus driving T4hard on. The condenser then discharges via the emittercollector path toground until the emitter again falls below the base voltage when T4switches off and the cycle again repeats. The values of the componentsof the oscillator circuit are preferably chosen such that a switchingfrequency at the base of transistor T1 of approximately 20 Hz isproduced. Each time the base of transistor T1 of is switched on, thecoil of relay RA is shunted, since point C is momentarily connected toearth. However, a time delay is incorporated in relay RA such that therelay cannot release even though the current through it consists ofimpulses.

Point C is connected through a capacitor C2 with a diode pump D9 to thebase of a transistor T5. Transistors T5 and T6 form a Darlington pairwhich maintain an indicating lamp L2 at full brightness providing anoscillating potential continues to appear at point C. Preferably thelamp 12 is green in colour to distinguish it from the orange lamp L1,and shows that the circuit is operating correctly. if any fail-dangerfault develops in the trip circuit, it will prevent the charging anddischarging of the capacitor C2 clue to cessation of impulsing by T1,and then lamp L2 is extinguished, indicating a trip unit fault. If lampL2 is extinguished, the potential at point Q rises to line potential andthis potential can be used to activate an audible fault warning alarm.If contact'Tl opens with the unit functioning correctly, point B risesto a steady DC potential, turning on transistor T1 as explained above,and the relaxation oscillator comprising transistor T4 is switched off.Point C is earthed and therefore the fault lamp L2 would go out.However, since the trip unit has functioned correctly, it is requiredthat the lamp L2 should remain on. This is achieved by restoring DCpotential to the base of transistor T5 via contact RA2 which closes whenthe relay RA is de-energised. The lamp L2 therefore remains illuminatedas required.

The connecting cable between the trip contact 0T1 and point A is alsomonitored by the self-checking facility. It is possible to produce afail-to-danger fault by either physically short circuiting the cable (asmay be caused by a spade), or by the ingress of moisture, creating asignificant earth leakage. It will be seen from the figure that point Ais held at approximately 3.5 volts when contact 0T1 is normally closed,the whole of the trip cable forming part of a potential divider. Point Ais connected to the base of a transistor T7, this being connected to atransistor T8 to form a Darlington pair which illuminates a cable faultwarning lamp L3 whenever 35 volts (or more) is supplied to the base oftransistor T7. [F contact 0T1 opens, the potential at point A rises to6.2 volts, which turns the transistor T7 hard on, and so'the lamp L3remains fully illuminated.

However, if a fault should develop along the trip cable causing either ashort circuit or a significant earth leakage, then the base oftransistor T7 becomes connected to earth potential and the cable faultwarning light L3 is extinguished. Point P rises to line potential whenthe lamp L3 is extinguished and this potential can be used to operate anaudible fault warning alarm. The lamp L3 may be green coloured as is thelamp L2, so that both are immediately recognisable as fault indicatinglamps in contradistinction to the lamp L1 which is orange coloured andindicates when relay RA is energised.

The lamps L2 and L3 are each shunted by a resistor, L2 being shunted byresistor R3 and L3 being shunted by resistor R4, so ensuring that anaudible warning will still be given in the case of a real faultoccurring, in spite of bulb failure in either or both of the lamps.

An open circuit fault occurring in the connecting cable between the tripcontact T1 and point A would normally be indistinguishable from theopen-circuit produced when contact 0T1 opens. However, although thisrepersents a fail-safe condition, it does not indicate that a fault hasoccurred and accordingly the relay may be de-energised unnecessarily. Toovercome this, a failsafe fault checking facility is incorporated intothe circuit shown.

A transistor T12 is connected in series with transistor T1. The base oftransistor T12 is connected by means of transistor T11 to a point nearpoint A. A resistor R is provided between point A and the base oftransistor T11, this resistor R5 being additionally connected to contact0T1. A further resistor R6 is connected across the contact 0T1, and boththe resistors R5 and R6 are wire wound.

While current flows from point A to earth, transistor T12 is held ON bytransistor T11. When the contact 0T1 is closed, point A is connected toearth by the combination of resistors R5 and R6, and when the contact0T1 has opened, point A is connected to earth through resistor R6 only.The condition of transistor T12 is constantly checked by theself-checking circuit. If transistor T12 is ON, the pulses appearing atthe base of transistor T1 cause transistor T1 to conduct, producing apulse signal at point C, and therefore the lamp L2 is lit. However, iftransistor T12 turns OFF, the pulse signal does not appear at point C,and hence lamp L2 is turned off, thereby indicating that a fault hasoccurred.

If one or both cores of the trip cable are severed, producing an opencircuit condition, current from point A to the contact 0T1 ceases,transistor T12 is turned off, and therefore the lamp L2 is extinguished,and the audible alarm is sounded. However, the relay RA is unaffectedand remains energized.

The circuit described above has the facility that both fail-safe andfail-danger faults occurring between contact 0T1 and relay RA do notrelease the relay RA but give either trip unit or trip cable faultwarnings. However, theself-checking circuit ensures that the uniquecondition of contact 0T1 opening must de-energise relay RA. It istherefore not necessary to use three trip units to guard againstfail-safe fault conditions to avoid a non-genuine trip de-energising therelay. Such guard systems are known and operate on a majority basis, butare complicated and expensive.

I claim:

1. A signal reproducing switching device having a plurality ofcomponents and comprising a solid state oscillating and switchingcircuit having a switched-on state and a switched-off state and normallyalternating between said states to produce a series of pulses, inputterminals and output terminals connected to said circuit and means forholding, upon arrival of an input signal at said input terminals, saidcircuit in one of said states, means connected to said output terminalsfor creating a signal at said output terminals corresponding to saidinput signal when said circuit is held in said one state by said inputsignal, means connected to said circuit for holding said circuit in theother of said states on failure ofa component of said device, andindicating means connected to said circuit for indicating said failurewhen said circuit is held in said other state.

2. A device as recited in claim 1 wherein said circuit includes anoscillator which drives said solid state switching circuit intoalternately conducting and nonconducting states.

3. A device as recited in claim 1 wherein said indicating means is alamp normally maintained lit by pulsed current produced under control ofpulses arriving at said lamp over a first path, and a second pathilluminating said lamp when said solid state switching circuit is heldin said one state by arrival of a signal at said input terminals.

4. A device as recited in claim 1 and in which for producing said seriesof pulses said circuit includes a relay with at least one contactconnected in parallel with said solid state switching circuit, saidrelay remaining operative so long as said switching circuit isnon-conductive for at least part of the time.

5. A device as recited in claim 4 wherein said input signal is a removalof a short-circuit across said input terminals, said output signal beingproduced by the opening of a relay contact by said relay.

6. A device as recited in claim 1 wherein said input signal is derivedfrom a remotely situated contact connected to said device by a cable,said device including determine duration.

1. A signal reproducing switching device having a plurality ofcomponents and comprising a solid state oscillating and switchingcircuit having a switched-on state and a switched-off state and normallyalternating between said states to produce a series of pulses, inputterminals and output terminals connected to said circuit and means forholding, upon arrival of an input signal at said input terminals, saidcircuit in one of said states, means connected to said output terminalsfor creating a signal at said output terminals corresponding to saidinput signal when said circuit is held in said one state by said inputsignal, means connected to said circuit for holding said circuit in theother of said states on failure of a component of said device, andindicating means connected to said circuit for indicating said failurewhen said circuit is held in said other state.
 2. A device as recited inclaim 1 wherein said circuit includes an oscillator which drives saidsolid state switching circuit into alternately conducting andnon-conducting states.
 3. A device as recited in claim 1 wherein saidindicating means is a lamp normally maintained lit by pulsed currentproduced under control of pulses arriving at said lamp over a firstpath, and a second path illuminating said lamp when said solid stateswitching circuit is held in said one state by arrival of a signal atsaid input terminals.
 4. A device as recited in claim 1 and in which forproducing said series of pulses said circuit includes a relay with atleast one contact connected in parallel with said solid state switchingcircuit, said relay remaining operative so long as said switchingcircuit is non-conductive for at least part of the time.
 5. A device asrecited in claim 4 wherein said input signal is a removal of ashort-circuit across said input terminals, said output signal beingproduced by the opening of a relay contact by said relay.
 6. A device asrecited in claim 1 whErein said input signal is derived from a remotelysituated contact connected to said device by a cable, said deviceincluding means indicating disconnection, short-circuit and earth faultsin said cable.
 7. A device as recited in claim 6 wherein said meansindicating cable faults includes a potentiometer one component of whichis located across said contact, and a transistor combination normallymaintained conductive by said potentiometer to light a lamp whilst noconductor fault is present.
 8. A device as recited in claim 1 includinga delay circuit which prevents repetition of an input signal to saidoutput terminals if said signal is of less than a pre-determineduration.